docs(ci): move CI handoff into docs/ci/handoff (avoid main collision)

main independently wrote its own docs/handoff/20260615-handoff-70.md the
same day, so my global-sequence handoff-70 was an add/add conflict waiting
to merge. Relocate it to docs/ci/handoff/20260615-handoff-ci-01.md (its own
namespace, like docs/ci/adr) + a README index. main's handoff-70 is
untouched; the merge becomes conflict-free.

docs/ci/handoff/20260615-handoff-ci-01.md

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+# CI subproject handoff — 2026-06-15 (ci-01)
+
+First handover for the **CI / release subproject** (the `ci` branch). Kept in
+`docs/ci/handoff/`, a namespace separate from the project's global
+`docs/handoff/` session sequence so it can't collide with `main`'s numbering —
+the same split as `docs/ci/adr/`, and needed for the same reason: `main`
+independently wrote its own **handoff-70** this same day (just as it took
+**ADR-0049**), which would have collided.
+
+A dedicated infrastructure session that built the project's **entire CI/CD
+pipeline** on the self-hosted Gitea Actions runner — from nothing to a live
+gate plus a six-target cross-platform release. Net: the **CI** /
+`requirements.md` **TT5** item and **D1**/**D2** are now done; **D3** and a
+couple of TT5 tails remain. Decisions are recorded in the sibling ADR namespace
+**`docs/ci/adr/`** (ADR-ci-001/002/003).
+
+## §1. State at handoff
+
+**Branch:** `ci` (worktree). **`main` has been merged into `ci`** (commit
+`138e766`, clean — `ci` and `main` touched disjoint files) so the gate runs
+against current `main` before CI lands there. Working tree clean except the
+in-progress doc updates from this handoff. Pushes/promotion are the user's
+step.
+
+**Gate verified locally on the merged code:** `clippy -D warnings` clean;
+**`cargo test` 2488 passing / 0 failing / 1 ignored** (the long-standing
+`friendly` doctest). main's features came in with their tests (2424 → 2488).
+
+**Pipeline (`.gitea/workflows/`):**
+
+- `build-ci-image.yaml` — builds + pushes the CI image (`node:22-bookworm-slim`
+  + single-user nix + the flake's devShell pre-warmed) to the Gitea registry.
+  Triggers only on image-input changes (Dockerfile / flake / toolchain).
+- `ci.yaml` — the gate: `clippy -D warnings` + `cargo test`, branch pushes + PRs
+  (docs-only changes skipped).
+- `release.yaml` — on a `v*` tag: `test` → `build` matrix over the **four
+  non-macOS** targets via `cargo-zigbuild`, upload to the Gitea release.
+- `release-macos.yaml` — **workflow_dispatch** (tag input) on the Tart
+  Apple-Silicon runner (`runs-on: macos`): test → build both `*-apple-darwin`
+  → de-nix `libiconv` + ad-hoc re-sign → upload.
+
+**Verified live this session:** the 4-target release published **8 assets**
+(binary + `.sha256` each) for tag `v.0.0.0-citest3`; the macOS build was proven
+portable (system-only deps) + signed + launches on the runner.
+
+## §2. What was built (and the non-obvious bits)
+
+- **Nix flake** (ADR-ci-002, relocated from a would-be `main` ADR-0049): one
+  pinned toolchain (`1.95.0`) for dev *and* CI; `cargo-zigbuild` + `zig` (Linux
+  only) for the cross targets; `apple-sdk` on darwin.
+- **Runner facts** (ADR-ci-001): jobs run *inside* a container (`ci-public` →
+  `catthehacker/ubuntu`), so host nix is unreachable — hence the baked image.
+  The Mac runner is **host execution**; its label is `macos` (`:host` in the
+  registration is the act_runner backend, not part of the label).
+- **Cross-compile** (ADR-ci-003): `cargo-zigbuild` for the 4 non-macOS targets.
+  Windows needs an **empty `libsynchronization.a` stub** (`ci/winstub/`, wired
+  via `.cargo/config.toml`) — std links `-lsynchronization`, absent from
+  rust-overlay's toolchain + zig's mingw, but forwarded by `kernel32`.
+- **macOS** (ADR-ci-003 amendment): built on **real Apple hardware** (Tart), so
+  the SDK is fully licensed — no osxcross grey area. The darwin stdenv bakes a
+  `/nix/store` `libiconv` path into the binary; the build rewrites it to
+  `/usr/lib/libiconv.2.dylib` (`install_name_tool`) and re-signs ad-hoc
+  (`codesign -f -s -`; `install_name_tool` invalidates the signature, arm64
+  refuses unsigned). A guard fails the build on any remaining `/nix/store` dep.
+- **Cache hygiene (Mac):** the runner wipes the workspace each run, so cargo
+  `target/` never accumulates; the persistent nix store is bounded by
+  **generation** (record the devShell in a persistent profile, keep the 2
+  newest via `nix-env --delete-generations +2`, GC the rest). First sweep
+  reclaimed a ~3.8 GB one-time backlog of build scaffolding (source + build-only
+  deps, *not* re-installed toolchains).
+
+## §3. Immediate next steps (user)
+
+1. **Push `ci`** → the gate re-runs in CI (should be green; no image rebuild —
+   the merge didn't touch the flake/Dockerfile).
+2. **Promote:** `git checkout main && git merge ci` — a **fast-forward** (`ci`
+   already contains `main`) — then push `main`. CI goes live; `release-macos`
+   becomes dispatchable (workflow_dispatch needs the default branch).
+3. **First real release:** tag `v0.1.0` (auto-builds the 4 Linux/Windows
+   assets), then **dispatch `release-macos` for `v0.1.0`** with the Mac up (adds
+   the 2 macOS assets) → a full 6-binary release.
+4. **Cleanup:** delete the `v.0.0.0-citest*` test tags + their releases.
+5. **Runner-side:** add `min-free`/`max-free` to the Mac's `/etc/nix/nix.conf`
+   as a hands-off nix-store backstop.
+
+## §4. Known gaps / follow-ups
+
+- **Versioning is not wired into the binary** (flagged by the user). The release
+  **git tag is nowhere in the produced binary** — there is no `--version` flag,
+  no `CARGO_PKG_VERSION` use anywhere in `src/`, and the release workflows use
+  the tag only for the *release name* + *asset filenames*
+  (`rdbms-playground-<tag>-<target>`). `Cargo.toml` is a static `version =
+  "0.1.0"`, decoupled from the tag. So a `v0.5.0` tag yields a `…-v0.5.0-…`
+  asset whose binary knows nothing of "0.5.0". To fix later: add a `--version`
+  flag, and inject the tag at build time (e.g. a `build.rs` reading a
+  CI-provided env, or bumping `Cargo.toml` as part of tagging) so the binary and
+  the release agree.
+- **D3 packaging** — Homebrew / Scoop / winget / `cargo binstall` manifests
+  (asset naming is already binstall-friendly).
+- **TT5 tails** — Windows is build-only (no execution runner); Tier-4 PTY (TT4)
+  is unwired in CI.
+- **`fmt` gate** — deliberately off (tree isn't stock-`rustfmt`-clean); revisit
+  on `main`.
+- **Website → Cloudflare** deploy — the separate, simpler workflow, still to do.

docs/ci/handoff/README.md

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+# CI / Build subproject — session handoffs
+
+Handover notes for the **CI / release pipeline** work (the Gitea Actions
+workflows under `.gitea/`, the nix flake, the release tooling). Kept in their
+own namespace, separate from the project-wide session handoffs in
+[`docs/handoff/`](../../handoff/), so a CI-branch handoff never competes with
+`main`'s global handoff sequence for numbers — the same split the CI ADRs use
+([`docs/ci/adr/`](../adr/README.md)). This is not hypothetical: `main`
+independently wrote a `handoff-70` the same day this subproject's first handoff
+was drafted.
+
+**Numbering.** Files are named `<date>-handoff-ci-<NN>.md` and referenced in
+prose as `handoff-ci-NN`. Assign the next free `NN` from this index.
+
+## Index
+
+- [handoff-ci-01 — the CI/release pipeline build-out](20260615-handoff-ci-01.md)
+  — Gitea Actions gate (clippy + test) + a six-target release (four via
+  `cargo-zigbuild` on a `v*` tag, two macOS via dispatch on a Tart runner), all
+  on a nix flake; decisions in `docs/ci/adr/`. Built on the `ci` branch, merged
+  `main` in, gate green (2488 tests), ready to promote to `main`.